1. Field of the Invention
The present invention relates to a feed safety apparatus for a movable member slidably supported on a stationary bed of a machine tool, such as a grinding machine.
2. Description of the Prior Art
In a grinding machine as disclosed in U.S. Pat. No. 4,115,958, a carriage rotatably supporting a grinding wheel is moved by a single servomotor at a rapid non-grinding feed speed until a programmed fast feed end point is reached and then at a slow grinding feed speed until a workpiece is finished to a predetermined size. If rapid feed movement of the carriage is continued beyond the programmed fast feed end point due to malfunction of the circuit which controls rotation of the servomotor, the grinding wheel would be brought into engagement with the workpiece at the rapid feed speed, and this results in breakage of the grinding wheel, fault of the workpiece and the like.
In order to prevent the grinding wheel from engaging the workpiece at the rapid feed speed, measures have been taken in the grinding machine to shut down the same in response to a signal from a sensor in the event that the current required by a motor to rotate the grinding wheel exceeds a predetermined relatively low idle current during rapid feed movement. However, if the sensor itself malfunctions, the grinding wheel could be still brought into engagement with the workpiece at the rapid feed speed.